Dense smog engulfed the Chinese capital, as well as a large swath of northern and eastern China, for a fifth day on Monday.
Data from the U.S. Embassy put levels of PM2.5 particles at 405 in Beijing as of 1 p.m. local time.
PM2.5 are tiny particulate matter less than 2.5 micrometers in size or about 1/30th the average width of a human hair. They can penetrate deep into the lungs, so measuring them is considered a more accurate reflection of air quality than other methods.
The U.S. Environmental Protection Agency considers levels above 300 to be hazardous.
The Beijing government raised its four-tiered alert system to "orange" for the first time on Friday after drawing public fire for its initial ineffective response.
The government introduced the tiered system last October, but despite several periods of thick smog since then, the plan's stronger measures have never before been introduced.
Government teams investigate pollution
On Monday, the country's weather bureau maintained the orange alert for seven provinces in northern and eastern China, covering 15 per cent of the country, state media reported.
The orange level, the second highest, advises schools and kindergartens to cancel sports classes conducted outside, but falls short of ordering schools to close and keeping government vehicles off the road, provisions which come into force with the "red" level.
Smoky air prompted health concerns among residents in Beijing, and many commuters were seen wearing masks.
Twelve teams of inspectors will head to the cities of Beijing, the nearby city of Tianjin and Hebei province to see how authorities are responding to the worst air pollution in months, the Ministry of Environmental Protection said on Sunday.
Chinese state broadcaster CCTV said the inspectors visited construction sites and factories producing steel, glass, cement and coal products. Those found to be violating production standards will be publicly identified, it added.
Authorities have issued innumerable orders and policies to try and clean up the environment, investing in projects to fight pollution and empowering courts to mete out stiff penalties. However, enforcement has been patchy at the local level, where authorities often rely on taxes paid by polluting industries.